Advice for prototype powered surfboard

[Felix de CatParticipant @felixdecat]

Super slim electric power assist for surfers

[Felix de CatParticipant @felixdecat]

Hello, Thanks for taking the time to read this! I really need some help and advice regarding my project. I am a keen surfer but as I grow older paddling out through the waves and paddling for a wave is getting harder.

 

My plan is to design and make a jet drive unit which I can fibre glass into one of my 9′ boards. I have calculated that I have about 40mm depth of thickness to work with and about 400mm length (this is the length of my vacuum former). The unit will need to hold as powerful a motor as I can fit and suitable prop. There will be a duct to allow the salt water to be drawn in and thrust out, resulting in thrust. I would love to achieve about 20 pounds of thrust for 10 second bursts. I hope to keep it simple, no ECU, just a 10 second timer and relay circuit.

 

My initial experiments have used adapted 12volt bilge pump motors with 40mm props. The bilge pumo is great as it can be submirged in salt water without problems but I am only getting about 2 pounds of thrust. I need more power!!

 

I need another idea. Is it best to use 2 or more smaller motors rather than one bigger on?. How much thrust could be realistic? Will there ever be enough to make it feasible? I guess there is because there is a product called Powrboard developed in America which looks amazing, trouble is it costs £3000. See the link http://www.youtube.com/watch?v=2BbE0rR4ADQ&feature=results_main&playnext=1&list=PLB43209205AC36546 another called wavejet  http://www.youtube.com/watch?v=tVk3D4AnnAY

 

Here is a close up of the wave jet unit.  http://wavejet.com/how-it-works/technology/the-wavejet-pod.html  I would like to develop a pair of slim units that I can fit  either side of the central stringer.

 

Should I use a special type of prop. The board will only travel at about 5 knots so is the fast model boat prop wrong for the task?

 

Should I try brushless? What about ECU problems and waterproofing. If so can you tell me of a waterproof brushless motor?

 

What battery type is best? NIMH seems good value but not as high energy density as lipo. Lipo seems a little high risk.

 

Can I waterproof a motor myself? I was considering encapsulating motors in epoxy or polyester resin. Will this seal the shaft enough to prevent water ingress? How does the bilge pump manufacturer seal the shaft?

 

I have so many questions I thought I would try to put the question out there to you guys as you have experienced so many of these things through your hobby.

 

Thanks for your responses

 

Regards from Cornwall

Phil

Edited By Felix de Cat on 24/11/2011 23:19:04

Edited By Felix de Cat on 24/11/2011 23:28:19

[ashley needhamParticipant @ashleyneedham69188]

felix. I think that 40mm depth of propulsive unit is insufficient to provide any meaningful power, prorp to fit this are just too small to do the job. Having several, ie 4 or 6 may do the job,but thats a lot of equipment..

 

A larger prop would be required…guessing.. 60mm minimum, two of. perhaps a jet type arrangement for safeties sake. Waterproofing the motor would be a problem, some sort of prop-shaft would be the way to go I think.

 

Would not like to use Lipo in this situation. Nimh can be made into flat packs easily enough.

 

Ashley

[Felix de CatParticipant @felixdecat]

Hello Ashley,

Thanks for your input. What size motor would be required to drive a 60mm diameter prop? Do you have any idea what pitch and rpm would be most suitable for a craft which will be traveling at about 5 mph?

 

I have the scope to house a larger prop by creating a bulge under the board to in the form of a tube.

 

I agree about lipo batteries but the energy density is tempting.

 

Regards

Phil

[ashley needhamParticipant @ashleyneedham69188]

jet props appear to have their own shape, and do need to be square at the edges to fit the tube, but I would imagine ordinary “X” pitch 2 blade plastic props may suffice. power….loads needed, sorry no idea. but several hundred watts per prop would likely be a minimum, perhaps 300?? dont really know, but needs to be substantial.

 

Lipo batteries…shorting out.. blowing up… oo er.. not a Lipo owner so only citing popular “common knowledge”. I am not sure weight its tooo much of an issue here, Lipo certainly deliver more for less as it were. i would take informed adviice first.

 

High speed mootors geared down 2 or 3;1 would be a more efficient way of driving (relative to model boat world) large props. Your project, considering the masses involved would need a good flow of water to produce sufficient thrust.

 

Ashley

 

 

[Felix de CatParticipant @felixdecat]

Hello,

 

Your comments are interesting. I think you may be right about 300 w. If I used 12 volts then I guess I need to look for a motor which is efficient at about 25 A. The graupner motors seem to offer the most information to make an informed choice.

 

Battery life may be very short but I guess a typical good surf may involve paddling into about 20 waves so it could be feasible with a 1000mA NIMH battery pack and 10 second bursts of power. Perhaps gym membership and an exta weatabix is the solution!

 

Thanks, I will keep designing!

[Colin BishopModerator @colinbishop34627]

Another way to look at it is to consider that the unit needs to propel you and the weight of the board. If you were in a lightweight dinghy you would be looking at a 2HP motor to give you 5 knots or so. Assuming you might get away with 1hp as there is no dinghy and the board would be much lighter you are looking at perhaps 1HP or 750w. maybe you could get away with 500w minimum.

 

For a guide on power consumption in this sort of range you could Google electric trolling outboard motors that fishermen use use and scale down from there. I think you are still going to need quite a lot of amps though and jet drives are not as efficient as a prop operating in open water (where it can do significant damage to your anatomy!).

 

I rather feel that if this idea is a practical proposition, somebody would have invented and marketed it already.

 

Colin

[Felix de CatParticipant @felixdecat]

Hello Collin,

I appreciate what you mean but there is no doubt it is totally doable, just look at the second link to a youtube clip on my initial posting. The Wavejet does it. It seems too good to be true but you can see it in action. They also claim a reasonable run time.

 

Now some may say, why not just order one and be done with it. But I am the sort of guy, like most people who enjoy model boats, that likes tinkering and thinks I could make one of those!

 

The wavejet clearly uses two water jet systems mounted in the middle of the board. I wish I could get one and take it to bits, but at the price they charge it is not going to happen unless I win the lottery.

 

I would love to know if anyone has experience of graupners jet drives and how much thrust they produce for a given voltage and current. I am sure someone must have experience.

 

Regards

Phil

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